The purpose of this project is to adapt the hydrogen(H2) clearance technique for measuring local tissue blood flow so as to measure local cerebral blood flow (LCBF) in fully-conscious cats and squirrel monkeys without use of any pharmacological immobilization whatsoever. In order to do so, the previously devised reference electrode in the form of a jeweler's screw affixed to the skull will replace the cumbersome calomel electrode (used by others). Measuring electrodes will be fashioned from insulated stainless steel neurological electrodes with 0.5 mm of the tip bared of insulation and platinized. Dental cement will be used to make a chronic preparation. The monkeys will be restrained in a primate chair and the cats in a rabbit box. Hydrogen will be administered by inhalation utilizing a head chamber and gas mixing chamber to control the atmosphere breathed. Measurements will be made daily or every few days for 1 to 2 months at 4 sites in the brain simultaneously. The effects of hypoxia, hypercapnia, anesthesia, tranquilizers, experimental subarachnoid hemorrhage and putative cerebrovascular dilators upon LCBF will also be studied.